The seals at present available for industrial purposes can be subdivided, in terms of their functions, into two main classes:
(a) Mechanical seals PA1 (b) Wet seals PA1 of the compression type (in which the seal is operated by compressing the packing with an external means); PA1 automatic (in which the seal is operated by using resilient elements which are positioned by the pressure of the same fluid which is to be restrained). PA1 floating (which include the types known in industry as "PISTON-RING" and "SEGMENTAL ROD PACKING"). These types are normally mounted in grooves and are spring-compressed. PA1 (a) Static seals PA1 (b) Dynamic seals PA1 Sealing--within the entire operational field (temperature, pressure, speed of rotation); PA1 Reliability--Possibility of checking the functioning of the individual parts and predicting their wear; PA1 Maintenance--Ease of maintenance and repair and simplicity of design; PA1 Length of operation--compatible with operational requirements; PA1 Cost efficiency--of design and maintenance; PA1 Autonomy--Low number of accessories or spare parts. PA1 (a) Leakage rate reduced to zero; PA1 (b) Absence of direct contact between the fixed part and the rotating part and absence of wear; PA1 (c) Reliability as regards contamination of the sealing liquids and leakage of the same; PA1 (d) Practically unlimited life; PA1 (e) Does not require auxiliary lubricating circuits; PA1 (f) Does not require periodic maintenance; PA1 (g) Is not affected by vibration or disalignment of the shaft. PA1 (a) Sodium pumps for fast reactors; PA1 (b) processing pumps treating fluids with a high level of toxicity; PA1 (c) machines for handling radioactive elements.
A more detailed description follows of the subdivisions within the two main classes.
Let us examine first of all the known mechanical seals:
(a) Packing seals which can be:
This type includes V-cup, U-cup, piston-cup and o-ring seals.
(b) Seals with (radial or axial) play.
In this type the difference in pressure between the two ambient media is reduced by means of a loss in load which is effected across the aperture separating the moving part from the fixed part. These types have a rather high leakage rate.
Special means have been developed which make it possible to improve this type of seal and which are based on the increase in loss of load between the parts in relative motion by adopting labyrinths, fixed or floating bushing etc.
(c) Sliding frontal seals (properly known as "mechanical seals")
This type consists of two parallel-faced rings, one fixed and the other one rotating which can also be slid in an axial direction. Sealing is ensured by the direct contact between the two parts. The contact force is determined by the resultant of the forces acting upon the sliding ring (pressure, inert weight etc.) as well as by the forces due to a series of springs. This system provides for various constructional solutions depending on the specific applications and functional requirements.
Wet seals ensure a sealing effect by interposing a certain quantity of liquid between the two ambient media.
This interposition can be effected in two different ways, giving rise to two main solutions:
In the first case ("hydraulic guards") only the static head is used as the balancing element for the difference in pressure between the two ambient media.
In the second case the sealing effect is ensured by injecting fluid in the aperture between the fixed part and the moving part. Some special means are, of course, necessary in order to limit the leakage rate.
These seals include the systems known in commerce under the names VISCOSEAL, ISO CARBON SEAL (of the Elliot Company), SIDERVALL etc.
Independently of the type used, the basic requirements to be satisfied are as follows:
The observance of these requirements is all the more necessary both with the increase in the dangerous nature of the fluid treated and with the importance of the machine in the cycle in which it operates.
These problems are accentuated in particular in the nuclear field where leakage rates, however small, cannot be permitted and where any maintenance or repair work is carried out in special operating conditions.
In the light of the above, the need arises to find solutions to the difficulties encountered in seals of the known type, as described above.
In hydraulic seals the separation between the fluid and the external ambient medium is achieved by interposing a liquid with a total head counteracting the difference in pressure.
Until now, this system has been used for mainly static applications; in fact appreciable speeds of rotation would involve a partial dragging of the interposed fluid and hence an increase in the level to the point of overflow.